WIKA T53 - Manual (Page 36)

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5.0 Link Active Scheduler (LAS)
5 .1 .Introduction:
T53 .features .a .LAS .function .which .is .only .available .in .Fieldbus .Foundation .installations . .
Please .note .that .the .LAS .function .has .not .been .specified .by .the .Profibus .Nutzerorganisation .
and .is .therefore .not .available .in .Profibus .installations .
5 . .Overview .
5 .3 .Description
All .links .have .one .and .only .one .Link .Active .Scheduler .(LAS) . .The .LAS .operates .as .the .bus .ar-
biter .for .the .link . .The .LAS .does .the .following:
• .recognizes .and .adds .new .devices .to .the .link .
• .removes .non-responsive .devices .from .the .link .
• .distributes .Data .Link .(DL) .and .Link .Scheduling .(LS) .time .on .the .link . .Data .Link .Time .is .a .
network-wide .time .periodicallydistributed .by .the .LAS .to .synchronize .all .device .clocks .
on .the .bus . .Link .Scheduling .time .is .a .link-specific .time .represented .as .an .offset .from .
Data .Link .Time . .It .is .used .to .indicate .when .the .LAS .on .each .link .begins .and .repeats .its .
schedule . .It .is .used .by .system .management .to .synchronize .function .block .execution .
with .the .data .transfers .scheduled .by .the .LAS .
• .polls .devices .for .process .loop .data .at .scheduled .transmission .times .
• .distributes .a .priority-driven .token .to .devices .between .scheduled .transmissions .
Any .device .on .the .link .may .become .the .LAS, .as .long .as .it .is .capable . .The .devices .that .are .
capable .of .becoming .the .LAS .are .called .link .master .devices . .All .other .devices .are .referred .
to .as .basic .devices . .When .a .segment .first .starts .up, .or .upon .failure .of .the .existing .LAS, .the .
link .master .devices .on .the .segment .bid .to .become .the .LAS . .The .link .master .that .wins .the .bid .
begins .operating .as .the .LAS .immediately .upon .completion .of .the .bidding .process . .Link .mas-
ters .that .do .not .become .the .LAS .act .as .basic .devices . .However, .the .link .masters .can .act .as .
LAS .backups .by .monitoring .the .link .for .failure .of .the .LAS .and .then .bidding .to .become .the .
LAS .when .a .LAS .failure .is .detected .
Only .one .device .can .communicate .at .a .time . .Permission .to .communicate .on .the .bus .is .con-
trolled .by .a .centralized .token .passed .between .devices .by .the .LAS . .Only .the .device .with .the .
token .can .communicate . .The .LAS .maintains .a .list .of .all .devices .that .need .access .to .the .bus . .
This .list .is .called .the .“Live .List .”
Two .types .of .tokens .are .used .by .the .LAS . .A .time-critical .token, .compel .data .(CD), .is .sent .by .
the .LAS .according .to .a .schedule . .A .non-time .critical .token, .pass .token .(PT), .is .sent .by .the .
LAS .to .each .device .in .ascending .numerical .order .according .to .address .